Power tongs are devices used to secure together ("make up") and detach ("break out") threaded ends of two adjacent tubular products such as pipe sections by gripping, applying torque to and rotating one of the sections. Other devices known as back-ups are often used in conjunction with such tongs to grip and hold against rotation the other of the two adjacent sections of pipe.
In recent years, major oil companies have required their drill string pipes or tubular products to be screwed and torqued together without damage to the tubulars so that stress and corrosion concentrations will not occur in the tubulars in the tears and gouges caused by the tongs and/or back-up teeth. In addition, to maintain integrity of the threaded connection it is desirable to reduce deformation of the pipe by the power tongs and back-ups near the location of the threads, thus allowing more compatible meshing of the threads and reducing frictional wear.
U.S. Pat. No. Re 31,993 (incorporated herein by reference for all purposes) issued on Oct. 1, 1985 as a reissue of U.S. Pat. No. 4,281,535 and describes means to accomplish the task of making and breaking the threaded joints of such tubular members.
The terms "pipe" or "tubular" as used herein shall include tubing and other cylindrical objects.
Gouging and tearing of pipe is caused in some instances by a number of undesirable conditions which cause concentration of the gripping force applied by the tong or back-up. For example, one such condition is insufficient contact area between gripping teeth of the tong or back-up and the pipe. Another is inadequate contact by one or more of a number of gripping members which engage the pipe, causing the gripping force to be concentrated with and applied by the remaining members. Still further, the gripping surface presented to the pipe may not conform in radius to the outer diameter of the pipe, causing uneven distribution of the gripping force across the surface of the pipe and concentrations at drastically reduced contact areas between the pipe and gripping mechanism.
Mis-alignment of the tong or back-up with the pipe may also cause gripping force concentrations leading to pipe damage. If the tong or back-up is not aligned axially with respect to the pipe the gripping surfaces may contact the pipe at an angle, thus causing the gripping force to be applied to the pipe along the edge of a gripping surface, for example. Typically, this damages the pipe because the pressure applied to the pipe is concentrated in the relatively small area of contact between the gripping mechanism and pipe instead of being spread over the entire face of the mechanism.
Considerable wear can also occur as the pipe is engaged by the tong. Many conventional tongs incorporate a gripping assembly including a mounting ring which supports a number of gripping members such as dies and rotates with respect to a drive ring. The gripping members are actuated by rotation of the drive ring to engage the pipe. At the outset of operation, the drive ring rotates to force the gripping members into contact with the pipe surface. However, because the mounting ring is also rotatable, rotation of the gripping members relative to the pipe generally occurs as the members are driven into secure engagement with the pipe. As a result, the gripping members of such mechanisms frequently cause scraping, gouging and marring of the pipe as the members slide across the pipe while being progressively tightened by the drive ring to a frictional stationary grip against the pipe prior to rotation.
At least one prior art device, more fully disclosed in U.S. Pat. No. 3,550,485 issued to J. L. Dickmann on Dec. 29, 1970, employs means for retarding movement of the mounting ring while the gripping members are actuated by the drive ring to engage the pipe. The retarding force applied to the mounting ring, however, must be manually activated and adjusted.
Because the retarding mechanism must be manually readjusted if deactivated, it is typically engaged constantly, even during make up and break out operations. As a result, wear of all associated parts is increased unnecessarily and the longevity of the device is substantially limited. If, on the other hand, the retarding mechanism is deactivated during operations, such devices become inherently inconsistent in the application of retarding force to the mounting ring due to repeated manual readjustment, frequently allowing the gripping members to skid across the pipe (if the retarding force is too low) or causing the gripping members to impact the pipe with too great a force (if the retarding force is excessive). In either case scraping, gouging and marring of the pipe typically results.
In conventional tongs, pressure applied by the gripping jaws is not distributed evenly around the pipe but is applied to areas spaced around the perimeter of the pipe. This causes undue deformation of the pipe as the jaws impinge against its surface. Since the jaws typically grip the pipe adjacent threads in couplings used to secure adjacent pipe sections, the threads deform with the pipe. Such deformation causes leaks across the threads, thereby reducing the useful life of the pipe.
The present invention overcomes these and other disadvantages associated with other tongs, back-ups and similar pipe gripping apparatus. The preferred embodiment is a pipe gripping apparatus for engaging, disengaging and/or rotating pipe sections which have gripping assemblies for gripping a section of pipe without slipping, marring, gouging or tearing the pipe. The gripping assemblies pivotally support a number of jaws. The jaws are actuated into and out of contact with the pipe during make up and break out procedures.
The radial gripping force applied to the pipe by the jaws is proportional to the torque applied to the pipe by the gripping apparatus so that the greater the torque applied, the greater the gripping force exerted by the jaws against the pipe. The surface area of the jaws contacting the pipe is sufficiently great, given the maximum gripping force to be applied to the pipe, to avoid reaching or exceeding the yield strength of the surface of the pipe. The contact surfaces of the jaws may, accordingly, be smooth or equipped with gripping teeth, or the gripping apparatus may incorporate jaws having both types of contact surfaces. The jaws are sized to replace gripping dies on existing state of the art gripping apparatus.
An additional feature of the preferred embodiment is a positioning device between the jaw members and the gripping assembly to which they are connected. This positioning device tends to hold the jaws, once moved to a retracted position out of contact with the pipe, in a position which will not interfere with insertion or removal of the pipe from the gripping apparatus. A further aspect of the invention is incorporation into the jaws of removable jaw sections or segments so that the entire jaw member need not be removed once the gripping teeth become worn or when different sized jaw members would otherwise be required to match pipes having different radii. These segments are configured to seat substantially flush against the pipe member to avoid force concentrations and slippage which would likely damage the pipe.
In one embodiment the jaw segments are attached to the jaw member in a manner allowing a slight play in the jaw teeth segment with respect to the jaw member so that the jaw segment can adjust itself slightly to make a relatively flush fit with the pipe. In one case, the jaw itself has a spherical connection to the gripping assembly which allows a slight pivoting movement of the jaw and its teeth along axes perpendicular to and parallel with the pipe and permits a relatively flush fit. In another embodiment a removable adapter link is coupled between the jaws and the gripping assembly so that adapter links of various sizes can be used to enable the gripping assembly to accommodate different sized pipes. This reduces manufacturing costs which would otherwise be needed for replacement jaw links and jaw members.
An additional feature is the provision of an adjustable drag on the drag ring to which the gripping assemblies are pivotally mounted. A drag force is applied to the drag ring to prevent it from rotating as the drive ring rotates to force the jaw members into engagement with the pipe. This causes the jaw members to grip the pipe with a predetermined radial force prior to the pipe being rotated. This effectively eliminates scraping, gouging and marring of the pipe which would otherwise be caused by slippage of the jaws over the pipe surface.
Projecting lugs are used on certain embodiments to position the jaws out of the way of the pipe when the jaws are retracted and rounded corners may be formed on the jaws to minimize damage caused by the jaw contacting the pipe on the edge of a gripping surface.
Another embodiment of the invention is an open throat tong utilizing three gripping assemblies rather than the two assemblies typically used by prior art devices. This causes the force applied to the pipe to be more uniformly distributed about the circumference of the pipe, distributes the force within the tong or back-up more evenly, lessens the force applied at any one point, and allows the associated jaws to more fully encircle the pipe. Jaws such as those previously discussed are pivotally mounted to the gripping assemblies.
The present invention also contemplates the use, in the alternative, of a special jaw link member pivotally coupled to the interface assembly. The link member has on one end a pivotal jaw and on the other end a jaw segment, both of which allow for misalignment and wear and provide a gripping surface for covering more of the pipe surface.